This invention relates to a synthetic method of an xcex1-ketol unsaturated fatty acid and, in particular, to improve the yield in a synthesis of an xcex1-ketol unsaturated fatty acid having a double bond at a xcex2-position to the ketone group thereof.
Heretofore, various actions of ketol fatty acids have been known in the art, and in recent years it has been reported that some of ketol fatty acids have a flower bud formation inducing effect (Japanese Unexamined Patent Publication Nos. 9-295908 and 11-29410).
Several attempts have been made for preparing ketol fatty acids efficiently. As in the case of xcex1-ketol unsaturated fatty acid (1) represented below, however, due to a double bond existing at a xcex2-position to the ketone group there is a problem that it is hard to be efficiently prepared by organic synthetic methods. 
As for examples of synthetic methods of xcex1-ketols, the following Reaction Formulae I to III may be provided. 
The Reaction Formula I shows a method to provide xcex1-ketol (12) by condensing acid chloride (10) and aldehyde (11) in the presence of samarium iodide (Tetrahedron Letters, Vol. 33, No. 19, 2621-2624 (1992), and Tetrahedron Letters, Vol. 35. No. 11, 1723-1726 (1994)). However, Reaction Formula I provided only a complicated mixture as a resulting product. Furthermore, the purification of such a product only gave compound (13) in which the double bond of xcex1-ketol (12) was transferred at low yield, so that Reaction Formula I could not allow the isolation of desired xcex1-ketol (12). 
The Reaction Formula II shows a method that dithiane (14) is converted into its lithio-derivative by a lithiation process using alkyl lithium such as n-butyllithium and then the lithio-derivative is condensed with aldehyde (11) to provide compound (15), followed by converting a dithio part of compound (15) into a carbonyl group to obtain an xcex1-ketol (12) (J. Org. Chem., Vol. 33, No. 1, 298-300 (1968), and J. C. S. Chem. Comm., 100-101 (1979)). In this case, however, the resultant was also a complicated mixture, so that the target compound could not be isolated therefrom. 
The Reaction Formula III shows a method using methylthiomethyl p-tolylsulfone derivative (16) which anions may present more stable than those of the dithiane. Namely, the methylthiomethyl p-tolylsulfone derivative (16) is reacted with aldehyde (11) just as in the case of Reaction Formula II to provide compound (17), followed by converting a thiomethylsulfone part of compound (17) into a carbonyl group to obtain xcex1-ketol (12) (Tetrahedron Letters, Vol. 27, No. 31, 3665-3668 (1986), and Tetrahedron Letters, Vol. 24, No. 51, 5761-5762 (1983)). In this method, compound (17) could be isolated but the yield was extremely low as 20% or less. Also, subsequent conversion reaction to a ketone gave a complicated mixture, so that the isolation was extremely difficult.
Thus, in the case of xcex1-ketol unsaturated fatty acid in which a double bond is present at a xcex2-position to the ketone, the yield of its skeleton-forming reaction is extremely low as the double bond may be easily transferred. This is the reason why it is hard to obtain the desired xcex1-ketol unsaturated fatty acid efficiently.
The present invention has been accomplished in view of the above problems of the prior art. It is, therefore, a object of this invention to provide a method for efficiently synthesizing xcex1-ketol unsaturated fatty acid having a double bond at a xcex2-position to the ketone.
As a result of the diligent studies conducted by the inventors for attaining the above object, it has be found that by a reaction between a monosubstituted acetylene and an epoxide a carbon skeleton of xcex1-ketol unsaturated fatty acid can be formed efficiently. Thus, the present invention has been accomplished.
Namely, a method of synthesizing an xcex1-ketol unsaturated fatty acid in accordance with the present invention comprises the steps of:
preparing compound (4) by reacting monosubstituted acetylene (2) with epoxide (3); and
preparing xcex1-ketol unsaturated fatty acid (1) from said compound (4), as shown in the following Reaction Formula 1:
xe2x80x83wherein
R1 represents an alkyl group of 1-18 carbon atoms or an aliphatic hydrocarbon group of 2-18 carbon atoms having 1-5 double or triple bonds at given positions;
R2 represents a protecting group for a hydroxyl group;
R3 represents a protecting group for a carboxyl group;
R is identical to R1 or, when R1 has one or more triple bonds, R represents an aliphatic hydrocarbon group in which each triple bond of R1 is converted to a double bond; and
A represents an alkylene group of 1-18 carbon atoms.
The method of the present invention preferably comprises the steps of:
reducing said compound (4) to produce compound (5);
oxidizing a hydroxyl group of said compound (5) to produce compound (6); and
deprotecting R2 and R3 of said compound (6) to produce said xcex1-ketol unsaturated fatty acid (1), as shown in Reaction Formula 2:
wherein R1, R2, R3, R, and A are the same as defined in said Reaction Formula 1.
Also, the method of the present invention preferably comprises the steps of:
reducing said compound (4) to produce compound (5);
deprotecting R3 of said compound (5) to produce compound (7);
oxidizing a hydroxyl group of said compound (7) to produce compound (8); and
deprotecting R2 of said compound (8) to produce said xcex1-ketol unsaturated fatty acid (1), as shown in Reaction Formula 3:
wherein R1, R2, R3, R, and A are the same as defined in said Reaction Formula 1.
In the present invention, R1 preferably represents R4xe2x80x94Cxe2x89xa1Cxe2x80x94CH2xe2x80x94, where R4 represents an alkyl group of 1-7 carbon atoms.
R4 preferably represents ethyl group.
xe2x80x9cAxe2x80x9d preferably represents an alkylene group expressed by xe2x80x94(CH2)nxe2x80x94, where n is an integer of 1 to 10.
xe2x80x9cnxe2x80x9d is preferably 7.
R2 preferably represents an ether-type protecting group.
The double bond of xcex1-ketol unsaturated fatty acid (1) preferably has a cis-configuration.
An intermediate for synthesis of xcex1-ketol unsaturated fatty acid (1) in accordance with the present invention is represented by the general formula (4): 
wherein
R1 represents an alkyl group of 1-18 carbon atoms or an aliphatic hydrocarbon group of 2-18 carbon atoms having 1-5 double or triple bonds at given positions;
R2 represents a protecting group for a hydroxyl group;
R3 represents a protecting group for a carboxyl group; and
A represents an alkylene group of 1-18 carbon atoms.
In a method of synthesizing an optically active xcex1-ketol unsaturated fatty acid in accordance with the present invention, it is preferably that in any methods mentioned above an asymmetric carbon atom of xe2x80x94C(OR2)xe2x80x94 in said epoxide (3) has either of R-configuration or S-configuration and that an asymmetric carbon atom in the xcex1-ketol structure of xcex1-ketol unsaturated fatty acid (1) has either of R-configuration or S-configuration.
Also, the method preferably comprises the steps of:
preparing compound (4A) by reacting said monosubstituted acetylene (2) with (R)-epoxide (3A) obtained from compound (21A) which asymmetric carbon atom at an aryl position has R-configuration; and
preparing (R)-xcex1-ketol unsaturated fatty acid (1A) from said compound (4A), as shown in the following Reaction Formula 1A: 
wherein R1, R2, R3, R, and A are the same as defined in said Reaction Formula 1.
Also, the method preferably comprises the steps of:
preparing compound (4B) by reacting said monosubstituted acetylene (2) with (S)-epoxide (3B) obtained from compound (21B) which asymmetric carbon atom at an aryl position has S-configuration; and
preparing (S)-xcex1-ketol unsaturated fatty acid (1B) from said compound (4B), as shown in the following Reaction Formula 1B: 
wherein R1, R2, R3, R, and A are the same as defined in said Reaction Formula 1.
An optically active intermediate for synthesis of said xcex1-ketol unsaturated fatty acid (1A) or (1B) is represented by the general formula (4A) or (4B): 
wherein
R1 represents an alkyl group of 1-18 carbon atoms or an aliphatic hydrocarbon group of 2-18 carbon atoms having 1-5 double or triple bonds at given positions;
R2 represents a protecting group for a hydroxyl group;
R3 represents a protecting group for a carboxyl group; and
A represents an alkylene group of 1-18 carbon atoms.
In Reaction Formula 1, R1 of monosubstituted acetylene (2) represents an alkyl group of 1-18 carbon atoms or an aliphatic hydrocarbon group of 2-18 carbon atoms having 1-5 double or triple bonds at optional positions therein.
The alkyl group may be either of a straight or a branched chain. Examples thereof include methyl, ethyl, propyl, butyl, pentyl, isopropyl, tert-butyl, hexyl, octyl, decyl, tetradecyl, octadecyl, 1-methylpropyl, 1-ethylpropyl, 3-methylbutyl, and 2-ethylhexyl. Preferably, the alkyl group has 1-10 carbon atoms.
The aliphatic hydrocarbon group having double or triple bonds may be either of a straight or a branched chain, where these multiple bonds do not limited to specific positions. The aliphatic hydrocarbon group has preferably 1-10 carbon atoms including one triple bond, and more preferably a group represented by R4xe2x80x94Cxe2x89xa1Cxe2x80x94CH2xe2x80x94. R4 represents an alkyl group of 1-7 carbon atoms, and preferably ethyl group.
A protecting group R2 for a hydroxyl group and another protecting group R3 for a carboxyl group in epoxide (3) are not limited, unless any trouble is caused in the synthetic method of the present invention. Examples of the protecting group R2 include ether-type protecting groups such as methoxymethyl (MOM), 2-methoxyethoxymethyl (MEM), tetrahydropyranyl (THP), 1-ethoxyethyl, tert-butyl, benzyl, trimethylsilyl (TMS), and tert-butyldimethylsilyl (TBDMS); ester-type protecting groups such as formyl, acetyl, and benzoyl; carbamate-type protecting groups such as benzyloxycarbonyl; and sulfonyl-type protecting groups such as p-toluenesulfonyl. Among them, it is preferably the ether-type protecting group, and more preferably MOM, MEM or TBDMS.
As for the protecting group R3, for example, methyl, ethyl, tert-butyl, benzyl, methoxymethyl (MOM), 2-methoxyethoxymethyl (MEM), or tetrahydropyranyl (THP) can be used.
In Reaction Formula 1, xe2x80x9cAxe2x80x9d represents a straight or branched alkylene group of 1-18 carbon atoms, preferably a straight alkylene group of 1-10 carbon atoms, and more preferably xe2x80x94(CH2)7xe2x80x94.
According to the synthetic method of the present invention, compound (4) is prepared as an intermediate by reacting monosubstituted acetylene (2) with epoxide (3), and then desired xcex1-ketol unsaturated fatty acid (1) is obtained from compound (4). xe2x80x9cRxe2x80x9d of xcex1-ketol unsaturated fatty acid (1) is derived from R1 of monosubstituted acetylene (2) used as a starting material. R may be identical to R1 or, if R1 has one or more triple bonds, an aliphatic hydrocarbon group in which such triple bonds are converted into double bonds.
In the present invention, unless otherwise specified, R1, R2, R3, R3, R, A, and n are defined as described above.
Although each double bond in xcex1-ketol unsaturated fatty acid (1) may be either of cis- or trans-configuration, in view of a effect such as a flower bud formation inducing effect or the like, a cis-configuration is preferable. Also, there is at least one asymmetric carbon atom in xcex1-ketol unsaturated fatty acid (1) of the present invention. The present invention includes each of optical isomers depend on the asymmetric carbon atom and a mixture thereof. In each synthetic step of the present invention, using well-known methods an optical resolution can be effected.
In Reaction Formula 1, the reaction between monosubstituted acetylene (2) and epoxide (3) can be performed by converting the monosubstituted acetylene (2) into its 1-lithio derivative with an organic lithium compound such as n-butyllithium or phenyllithium, and then by reacting the derivative with epoxide (3). If required, a Lewis acid such as boron trifluoride etherate or a base such as ethylenediamine or tetramethylethylenediamine may be added thereto. As for a solvent, tetrahydrofuran (THF), diethyl ether, dimethyl sulfoxide (DMSO), or the like can be used. The reaction is preferably performed at a low temperature of xe2x88x9250xc2x0 C. or less.
In Reaction Formula 1, the reaction for converting compound (4) into xcex1-ketol unsaturated fatty acid (1) includes reduction of triple bonds to double bonds, oxidation of a hydroxyl group, and deprotection of R2 and R3 in appropriate order. For example, As shown in Reaction Formula 2, after the reduction of triple bonds to double bonds, the oxidation of a hydroxyl group and the deprotection of R2 and R3 are performed successively. The order of deprotection of R2 and R3 is not limited and both R2 and R3 may be removed by deprotection at the same time depending on the species thereof. Also, as shown in Reaction Formula 3, it is possible to perform reduction of triple bonds to double bonds, the deprotection of carboxyl-protecting group R3, the oxidation of a hydroxyl group, and the deprotection of R2 in this order. However, Reaction Formula 2 is preferable.
For the reduction of compound (4), a selective reduction process should be selected depending on a cis- or trans-configuration of each double bond to be obtained. For example, a selective reduction from triple bonds to cis-double bonds can be performed by a catalytic reduction process using nickel acetate-NaBH4 as a catalyst or another catalytic reduction process using Pd-CaCO3 or Pd-BaSO4 as a catalyst in the presence of lead acetate or quinoline. As for a solvent to be used, alcohols such as methanol and ethanol, ethyl acetate, acetic acid, diethyl ether, benzene, hexane, dioxane, and the like can be used. The reaction temperature may be in the range of room temperature to a reflux temperature. Also, the above goal can be achieved by performing a hydroboration process with diborane to produce a corresponding vinyl borane derivative and then hydrolyzing the derivative with acetic acid or the like. Furthermore, it can be achieved by a reduction process heating a material in methanol together with a zinc-copper alloy.
A selective reduction to trans-double bonds includes a process using an alkali metal such as sodium, lithium, or potassium in an amine solvent such as liquid ammonia, methylamine, ethylamine, or ethylenediamine. As for a solvent in this case, in addition to the amine described above, an alcohol, diethyl ether, THF, dimethoxyethane (DME), and the like can be used.
Any reaction for forming a carbonyl group of xcex1-ketol in conversion from compound (5) into compound (6) or from compound (7) into compound (8) can be performed by an oxidation of a hydroxyl group. Such oxidation may be chromic acid oxidation, DMSO oxidation, or an oxidation using dimethyl sulfide (DMS)/N-chlorosuccinimide (NCS), o-iodoxybenzoic acid, or the like.
In the chromic acid oxidation, chromic compounds including chlomium oxide (VI), a dichromate such as potassium dichromate or sodium dichromate, pyridinium chlorochromate (PCC), pyridinium dichromate (PDC), or pyridinium fluorochromate (PFC) can be used. In the case using chromium oxide (VI) or a dichromate, for example, under an acidic condition with sulfuric acid the reaction can be carried out in a solvent such as acetone, acetic acid, THF, dioxane, diethyl ether, benzene, chlorobenzene, or carbon tetrachloride. Also, there is an another method using chrome oxide (VI)-pyridine complex in a solvent such as pyridine or dichloromethane. In the case using a chrome compound such as PCC, PDC, or PFC, the reaction can be carried out in a solvent such as dichloromethane. In any event, the reaction temperature is typically in the range of 0 to 60xc2x0 C.
In the DMSO oxidation, the reaction is typically carried out in the presence of an appropriate electrophilic reagent. Depending on a kind of the electrophilic reagent used, it can be broadly classified as DMSO-dicyclohexylcarbodiimide (DCC) method, DMSO-acetic anhydride method, DMSO-phosphorus pentoxide method, DMSO-sulfur trioxide-pyridine method, and so on. In the case of DMSO-DCC method, for example, additionally using pyridine-trifluoroacetic acid is used as a hydrogen donor, the reaction can be carried out in a solvent such as benzene if required. The DMSO-acetic anhydride method is generally carried out without any solvent. The DMSO-phosphorus pentoxide method is carried out in a solvent such as dimethylformamide (DMF). The DMSO-sulfur trioxide-pyridine method is carried out in the presence of trimethylamine. In each of the cases, typically, the reaction temperature may be in the range of room temperature to 70xc2x0 C.
In the DMS/NCS oxidation, it is preferable that a DMS-NCS complex is prepared in toluene at 0xc2x0 C. and then subjected to the oxidation process at 0xc2x0 C. to xe2x88x9225xc2x0 C.
In the oxidation using o-iodoxybenzoic acid, a solvent may be DMSO, a halogenated hydrocarbon such as chloroform or dichloromethane, an aromatic hydrocarbon such as benzene, xylene, or toluene. The reaction temperature is typically in the range of room temperature to a reflux temperature.
A deprotecting reaction of R2 to be required for obtaining xcex1-ketol unsaturated fatty acid (1) as a final product can be performed by well known methods such as described in xe2x80x9cProtective groups in Organic Synthesisxe2x80x9d, T. W. Greene, and P. G. M. Wuts, John Wiley and Sons, Inc., and so on, depending on a selection of the protecting group. If R2 is TBDMS, for example, the deprotecting reaction can be carried out in a solvent such as acetonitrile, or water with hydrogen fluoride.
The deprotection of R3 also can be performed by well known methods, depending on a selection of the protecting group. In the case where R3 is an alkyl group such as methyl group, for example, the deprotection can be performed by a hydrolysis using a base such as potassium hydroxide or a sodium hydroxide in a solvent such as methanol or water. In addition, it is possible to perform the deprotection with an enzyme such as a lipase.
In each of the conventional methods represented by the respective Reaction Formulae I to III as described above, a carbon skeleton of xcex1-ketol unsaturated fatty acid (1) is formed by a condensation reaction between a carbon of a carbonyl group and a carbon of Cxe2x80x94OH group. In the synthetic method of the present invention, on the other hand, an epoxide (3) having a 1,2-epoxy-3-hydroxy structure as a precursor of xcex1-ketol is reacted with monosubstituted acetylene (2) to form a carbon skeleton of xcex1-ketol unsaturated fatty acid. According to the present invention, compound (4) to be an intermediate can be obtained in a high yield of 85% or more and following steps are convenient, so that xcex1-ketol unsaturated fatty acid (1) can be efficiently prepared as a final product.
The monosubstituted acetylene (2) of Reaction Formula 1 can be commercially available or prepared by a well known reaction process. For instance, 1,4-heptadiyne can be prepared by reacting 1-butyne with ethyl magnesium bromide to produce a corresponding magnesium acetylide and then reacting the latter with 3-bromopropane-1-yne (e.g., Japanese Unexamined Patent Publication No. 53-34926, and J. Chem. Ecol., 4, 531-542 (1978)). Another monosubstituted acetylene can be prepared in the same way. Furthermore, it can be prepared by a dehydrohalogenation reaction of 1,2-dihalogenoalkane or halogenoalkene with a base, or a dehlogenation reaction of tetrahalogenoalkane or dihalogenoalkene.
The epoxide (3) can be also prepared by a well known reaction. A representative synthetic example will be shown bellow. 
In Reaction Formula 4, the epoxide (3) can be obtained by: preparing an aldehyde (11) by a selective reduction of dicarboxylic acid halfester (20-1) or by a reductive ozonolysis or a periodate degradation of compound (20-2); converting a formyl group of aldehyde (11) into a H2Cxe2x95x90CHxe2x80x94CH(OH)xe2x80x94 group; performing an epoxidation of a double bond; and then protecting a hydroxyl group. Although R5 of compound (20-2) may be any substituent unless otherwise effected on the reaction, it is preferably an alkyl group.
The aldehyde (11) of Reaction Formula 4 can be obtained by each of several methods. For example, the aldehyde (11) may be obtained by reacting dicarboxylic acid halfester (20-1) with 1,1-carbonyldiimidazol to obtain an acid imidazolide and then reducing the latter with lithium aluminium hydride tri-tert-butoxide. These reactions can be typically performed in an anhydrous solvent such as diethylether or THF at a temperature in the range of 0xc2x0 C. to a reflux temperature.
Alternatively, aldehyde (11) can be obtained by reacting compound (20-2) with ozone to obtain an ozonide and then reducing the latter with dimethylsulfide or the like. This reaction can be typically performed in an organic solvent such as methanol at a temperature in the range of xe2x88x9280xc2x0 C. to 0xc2x0 C. As another method, furthermore, aldehyde (11) can be obtained by epoxidation of compound (20-2) with a peracid such as m-chloroperbenzoic acid and then reacting the resulting epoxide with a periodic acid. The epoxidation is typically performed in an organic solvent such as dichloromethane, hexane, ethyl acetate, diethylether, or methanol at a temperature in the range of 0xc2x0 C. to a reflux temperature. The reaction with the periodic acid is typically performed in an aqueous organic solvent such as a mixture of dioxane with water at a temperature in the range of 0xc2x0 C. to a reflux temperature.
In a second stage, for example, by Grignard reaction using vinyl magnesium bromide aldehyde (11) can be converted into compound (21). This reaction is typically performed in an anhydrous solvent such as diethylether or THF at a low temperature of xe2x88x9230xc2x0 C. or less.
In a third stage, for example, compound (21) can be converted into compound (22) by epoxidation with a peracid. The peracid may be perbenzoic acid, m-chloroperbenzoic acid, peracetic acid, hydrogen peroxide, or the like. The solvent may be selected from hydrocarbons such as benzene and hexane, halogenated hydrocarbons such as dichloromethane and chloroform, esters such as ethyl acetate, ethers such as diethyl ether and THF, alcohols such as methanol, and so on. The reaction temperature is typically in the range of 0xc2x0 C. to a reflux temperature.
In a fourth stage, by protecting a hydroxyl group of compound (22) a desired epoxide (3) can be obtained. The protecting reaction can be performed in a conventional manner depending on a selection of the protecting group (e.g., xe2x80x9cProtecting groups in Organic Synthesisxe2x80x9d described above). If the protecting group is TBDMS or TMS, for example, the corresponding chlorosilane compound is used for the reaction in the presence of a base such as pyridine, triethylamine, triethanolamine, urea, DBU, or imidazole. The reaction is typically performed in a solvent such as benzene or DMF at a temperature in the range of room temperature to a reflux temperature.
The optical isomer (1A) or (1B), in which the asymmetric carbon atom of xcex1-ketol structure xe2x80x94COC(OH)xe2x80x94 in xcex1-ketol unsaturated fatty acid (1) has R- or S-configuration respectively, can be obtained by optical resolution of xcex1-ketol unsaturated fatty acid (1) prepared in the above Reaction Formula 1. Alternatively, as shown in Reaction Formula 1A or 1B, it can be synthesized by a reaction according to Reaction Formulae 1 to 3 using an optically active epoxide (3A) or (3B). The epoxide (3A) or (3B) is an optical isomer in which the carbon at 3-position of 1,2-epoxide-3-hydroxy structure being a precursor of xcex1-ketol structure has R- or S-configuration. The epoxide (3A) or (3B) can be synthesized using a well known reaction. For example, according to the third to fourth stages of the above Reaction Formula 4, it can be prepared from an optically active compound (21A) or (21B) in which an asymmetric carbon atom of xe2x80x94C(OH)xe2x80x94 is a R- or S-configuration.
The compound (21A) or (21B) can be obtained from racemic compound (21) thereof by a well known method. For example, a direct optical resolution, which may be performed by a liquid chromatography using an optically active column, can be used. Also, a method including: binding the compound (21) with an optically active compound by an ester bond or the like to induce compound (21) to a diastereomer; separating the diastereomer by a well known process such as recrystallization, thin-layer chromatography, or liquid chromatography; and then breaking the bond can be used. Furthermore, one of optical isomers (21A) and (21B) is reacted by an optically selective enzymatic reaction to compound (21) and is removed as a reaction product, thereby obtaining the other optical isomer.
As a representative example, the racemic compound (21) is selectively reacted with a vinyl acetate using an enzyme such as a lipase to acetylate either the optical isomer (21A) or (21B), and then a separation between a resulting acetylated isomer and an unacetylated isomer is carried out, thereby obtaining an optical isomer (21A) or (21B). The present reaction is typically performed in an organic solvent such as pentane or diisopropyl ether at a temperature in the range of xe2x88x9240xc2x0 C. to 40xc2x0 C.
Also, the isomer (21A) or (21B) can be obtained by a deacetylation of the acetylated isomer using a well known method. For example, there are methods for removing an acetyl group by an enzyme such as esterase or lipase, or by treatment with a base such as sodium hydroxide or potassium hydroxide. In this case, if a protecting group R3 for a carboxylic acid at an end of the compound (21A) or (21B) is removed, the carboxyl group may be protected by an appropriate protecting group R3 such as methyl group to obtain the compound (21A) or (21B).
Hereinafter, but not limited to, the present invention will be described by examples. 